v1.0.18

fix(jetson): faster, sustain-based fan check

The pwm-fan thermal governor rewrites pwm1 on its own ~1s cycle, so the old
ramp test's stabilisation sleeps (~17s) only ever sampled governor-reasserted
RPM, not the commanded response — which is why its ramp comparisons were
warnings, never failures. It also read idle RPM, which is 0 when the governor
parks a cool Jetson's fan.

Hold the fan at full by re-writing pwm1 faster than the governor reasserts, then
require RPM to stay above threshold across a ~3s sustain window. ~3.8s instead of
~17s, and a stronger check: a fan that spins up then seizes now fails where the
peak-only ramp passed it. Validated on a bench Jetson (healthy fan holds
~5000 rpm; a spin-up-then-drop case fails at the sustain floor).